In the microelectronics industry, the requirements for increased circuitry for a given chip size has driven to smaller half-pitch nodes in order to increase the process speed and improve the chip efficiency. Microlithography techniques are key to reducing the size of structural features. The depth of focus and resolution depend on the numerical aperture of the lithography apparatus and the wavelength of light.
As feature sizes continue to decrease in size, it is particularly difficult to pattern round via holes or contact holes into photo-resists. As features get smaller and approach 50 nm and beyond in diameter this task becomes extremely difficult to achieve a smooth round shaped via hole of the appropriate dimension in relatively thick film of photoresist. Therefore, alternative approaches to forming via or contact holes in a more etch resistant film are desired.
In this invention a “reverse” patterning technique is employed. The reverse pattern technique involves using a photoresist to pattern posts into the photoresist instead of via holes. After the appropriate dimension of the post is patterned using a photoresist, a silicon containing material is coated on top of the patterned post to cover the entire pattern. For application, this silicon resin is carried in a solvent that the photoresist is not soluble in. After coating, the silicon containing film will be subjected to a low temperature prebake to drive off solvent and slightly crosslink the resin. In some cases further cure is required and can be performed with some activator under thermal or UV treatment. In order to transfer the reverse pattern of the photoresist into the silicon containing resin, two dry etching techniques are employed. The first etch step is to use a reactive ion etch recipe containing CF4 to “etch back” the silicon resin to the top of the photoresist material, exposing the entire top surface of the photoresist. The second etch step is to use a second reactive ion etch recipe containing O2 to etch away the photoresist. The result is a silicon resin film with via holes with the size and shape of the posts that were patterned into the photoresist. The resulting film can be used to further transfer the via hole pattern onto another critical layer. Since it is much easier to pattern post in a photoresist than via holes, this process provides a way to pattern a more etch resistant silicon containing resin than alternative approaches. In this document this approach is referred to as “reverse patterning.”
In order for a silicon containing material to be useful in reverse patterning, it must meet several criteria. First, it must be carried in a solvent such as an organic alcohol or ether that the photoresist is not soluble in. Additionally, it must be capable of being cured by multiple cure methods to allow for the formation of select compositions that can be etched by different etch methods such as CF4 and O2. It has been found that certain silsesquioxane resins meet these criteria.